With recent reduction in the size and weight of electrical products, lithium ion secondary cells having high energy density have been developed. Also, with an expansion of the field of application of the lithium ion secondary cells, improved cell characteristics have come into demand. Especially, the safety and cell characteristics will become increasingly important in using lithium ion secondary cells in vehicles.
Lithium ion secondary cells are, however, insufficient in safety when they are overcharged, when they have an internal short circuit, or when they are penetrated by a nail. In the case of in-vehicle use, higher safety is required. Also, in the case of in-vehicle use, the voltage is required to be higher than that currently used to increase the capacity.
As a method for providing a nonaqueous electrolyte secondary cell with improved safety and a higher voltage, use of a fluorine-containing ether having a specific structure has been proposed (see Patent Literature 1, for example). The nonaqueous electrolyte secondary cell of Patent Literature 1, however, is disadvantageous in that the discharge capacity is reduced when it is left in a high temperature environment or when repeatedly charged and discharged.